Glissando is a musical effect in which a transition is made from a first note to a second note by playing a sequence of intermediate notes. In a keyboard instrument such as a piano, the notes are played by running the finger over either the intermediate white keys or intermediate black keys. Thus the intermediate tones correspond to the diatonic tone intervals of the musical scale. However, to play a chromatic glissando in which all the intermediate black and white keys are successively operated is extremely difficult to play rapidly since it requires extreme manual dexterity to accomplish, such as a fast full chromatic transition between two designated notes.
The present invention is directed to a circuit for producing a chromatic glissando effect in a keyboard-operated instrument of the type described in my copending application Ser. No. 603,776, filed Aug. 11, 1975, now issued as U.S. Pat. No. 4,085,644 and entitled "Polyphonic Tone Synthesizer". In the musical instrument therein described, the waveform of each note is controlled by data calculated and stored in a master data list, the data being converted by a digital-to-analog converter to an analog voltage for driving a conventional audio sound system. The fundamental frequency of the waveform is determined by a voltage-controlled oscillator, the input control voltage to oscillator in turn being determined by addressing a frequency control number from a frequency number table and converting the frequency number to a corresponding voltage level. A particular key on the keyboard determines the address of the frequency number stored in the number table, all in a manner more specifically described in copending application Ser. No. 634,533, filed Nov. 24, 1975, now issued as U.S. Pat. No. 4,067,254 and entitled "Frequency Number Control Clocks" and U.S. Pat. No. 4,022,098 entitled "Keyboard Switch Detect and Assigner".
In implementing the glissando effect in the present invention, techniques similar to those described in my copending application Ser. No. 718,791, filed Aug. 30, 1976, entitled "Constant Speed Portamento", assigned to the same assignee as the present application and now U.S. Pat. No. 4,103,581 herein incorporated by reference, are employed. The portamento effect is similar to the glissando effect in that the transition between two notes is characterized by the sliding of the pitch of each note in a smooth transition to the pitch of the next note as successive keyed notes are played on the keyboard. The portamento effect differs from the chromatic glissando effect in that in the portamento effect of the pending application, the transition takes place over a time interval which is independent of the difference in frequency of the successively keyed notes. The incremental frequency steps involved in the transition are a fixed ratio of the difference in frequency between the two notes. Thus there is no relationship between the incremental steps and the steps of the chromatic scale, for example. In contrast, in the glissando effect the transition is made by sounding each of the individual notes in the chromatic scale in the interval between the beginning and ending keyed notes, so that the time of transition is a function of the frequency separation between the two keyed notes. The audible effect is therefore considerably different. The transition time in the glissando effect can be made sufficiently slow so that each individual note of the chromatic scale can be audibly distinct, if desired. Moreover, each of the transition notes can be sounded with the same attack, decay, sustain and release characteristics of the starting and ending notes.